Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

SYSTEM DESCRIPTION
The Exhaust Gas Recirculation (EGR) system routes exhaust gases from the exhaust manifold to the intake manifold at specified engine operating conditions. The EGR valve is opened by ported vacuum applied to the top side of the diaphragm. This vacuum must overcome spring pressure in order to open the exhaust gas port. The exhaust gases blend with the air/fuel mixture into the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber during EGR operation, the temperature is reduced and Oxides of Nitrogen (NOx) formation is limited. If too much exhaust gas enters, combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, none at idle, wide open throttle, or during high engine load conditions. The valve should only open when the engine is at operating temperature and above base idle speed. The amount of exhaust gas flow is controlled by variations in engine vacuum and exhaust back pressure.

Negative Back Pressure EGR Valve:

Positive Back Pressure EGR Valve:

EGR TYPES
Positive Back Pressure Type
The Positive Back Pressure EGR valve has a bleed valve that controls the amount of vacuum in the vacuum chamber. The bleed valve allows the vacuum to vent to atmosphere during open phase. When the bleed valve receives sufficient exhaust back pressure through the hollow shaft, the bleed valve closes. The maximum amount of vacuum is available once the bleed valve closes, which allows the EGR valve to open. If there is no exhaust back pressure present, or if the throttle is wide open or at base idle, then the EGR valve is closed. The amount of exhaust gas flow is determined by manifold vacuum and throttle valve opening.

NOTE: The positive back pressure EGR valve will not open when vacuum is applied to the valve, with the engine not running.

Negative Back Pressure Type
The Negative Back Pressure EGR valve has an internal vacuum bleed hole which is forced closed by a small spring when no exhaust back pressure is present. Vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with exhaust back pressure, the vacuum bleed hole opens and the EGR valve closes.

NOTE: The negative back pressure EGR valve will open if vacuum is applied to the valve, with the engine not running.